Large supercharged diesel engine

ABSTRACT

Exhaust gas in a supercharged internal combustion engine is recycled from the high pressure side of the turbocharger to the charging air system of the engine. At least part of the recycled exhaust gas is humidified to largely 100 percent relative humidity. This on one hand cools the gas to that the temperature at initiation of the combustion in the engine cylinders is lowered, and on the other hand the heat capacity of the steam in the gas restricts the temperature raise occurring during the combustion. These factors both act to reduce the amount of NO x  produced by the combustion. The addition of water is effected by a scrubber (16) which purifies the recycled gas. A blower (17) augments the pressure of the recycled gas. The scrubber may be made to generate fresh water by designing it with several stages where sea water is supplied to the first stage and fresh water to the last stage.

The invention relates to a large supercharged diesel engine, such as amain engine of a ship, comprising a turbocharger having a turbine drivenby exhaust gas and a compressor driven by the turbine and supplyingcharging air to the cylinders of the engine, and a recycling passage forreturning part of the exhaust gas to the engine cylinders.

Japanese patent publication No. 53-5321, for example, describes gasolineengines, wherein the NO_(x) content of the exhaust gas has beenrestricted by recycling part of the exhaust gas to the intake system ofthe engine so that the maximum combustion temperature of the enginebecomes lower. The amount of NO_(x) produced by the combustion dependson the maximum combustion temperature in that an increasing temperatureleads to a sharp increase in the amount of NO_(x). Recycling of 5-20percent of the exhaust gas may result in a reduction of up to 30percentof the amount of NO_(x) in the exhaust gas, using prior art.

Naturally, it is desirable to restrict the engine emission ofenvironmentally harmful compounds at source, but the known method forrecycling exhaust gas is not without its problems. For example, therecycling tends to increase the amount of particles in the exhaust gas,which is visible as smoke. This is probably due to the fact that theoxygen content of the charging air drops with the increased degree ofrecycling.

DE-A 25 04 308 describes an engine wherein part of the exhaust gas ispurified by bubbling through a water bath before the recycling to theintake side. It is stated that the purified gas contains steam whichdoes not contribute to lowering the NO_(x) content, but instead resultsin reduced fuel consumption.

DE-C 41 23 046 describes an engine which by use of a pump or compressorpresses recycled exhaust gas through a material with fine pores so thatthe gas passes through a water bath as finely distributed bubles. Theengine is not supercharged.

It is known from WO 88/01016 to purify the exhaust gas from a dieselengine in a water bath and then recycle part of the purified andhumidified gas to the intake side of the engine. The humidification inthe water bath results in a certain cooling of the exhaust gas andcontributes to keeping down the maximum combustion temperature. As thesteam has a large specific heat capacity, the effect of recyclingexhaust gas is greater than in the above methods. The engine is notsupercharged, and the water consumption is large, because all theexhaust gas is humidified.

Large diesel engines normally use heavy fuel oil as fuel, which resultsin large amounts of combustion products which are very aggressive to theengine components in contact with the exhaust gas. The heavily pollutingexhaust gas cannot be sufficiently purified by bubbling through a waterbath. It is known from DT-A-24 43 897 to pass exhaust gas to the intakeside of a diesel engine in order to reduce the NO_(x) formation duringcombustion. To avoid the impurities from the diesel engine's own exhaustgas, the exhaust gas from a gasoline engine is used instead as therecycling gas for the diesel engine. This solution renders the engineplant substantially more complicated and is also inapplicable in a shipwhere normally only a very limited amount of fuel in the form ofgasoline is allowed.

SE-B 314 555 and U.S. Pat. No.4,440,116 describe turbo-charged engineswherein water is injected in the intake air upstream of the compressorto limit the temperature increase of the intake air during compression.In large high-powered diesel engines, such a water addition may resultin disadvantageously rapid corrosion of the sensitive compressor, aseven a very small amount of fine drops of water can erode the blades ofthe compressor wheel.

The object of the invention is to enable recycling of exhaust gas in alarge supercharged diesel engine in such a manner that the enginemaintains high efficiency and a long life for the engine components,particularly expensive components, such as turbochargers, and that thecomponents of the recycling system are of small dimensions advantageousto the integration with the engine.

With a view to this object, the above large diesel engine is accordingto the invention characterized in that the recycling passage includes aunit for humidification of the exhaust gas with water, that thehumidification unit is a scrubber having a number of water atomizingstages, that the recycling passage comprises a conduit which branchesoff from the exhaust conduit upstream of the turbine and is connectedwith the charging air conduit downstream of the compressor, and that theconduit is connected with the scrubber and with a blower, preferablypositioned downstream of the scrubber, for augmentation of the pressurein the recycled exhaust gas.

By using a scrubber having at least one water atomizer stage forpurification of the exhaust gas, the purification becomes so efficientthat the diesel engine's own exhaust gas may be recycled even when theengine is run on heavy fuel oil. The scrubber does not require muchspace, and the arrangement of the atomizer nozzles of the scrubber inthe flow passage of the exhaust gas only results in an advantageouslysmall pressure drop over the scrubber, which is of importance for themaintenance of high engine efficiency. Over a very short gas flow lengththe scrubber is able to atomize very large amounts of water into therecycled gas, which is of importance for obtaining the desired goodpurification, efficient cooling and humidification to largely 100percent relative humidity. The scrubber yields a higher cooling of therecycled gas than the known humidifiers, which promotes the reduction ofthe formation of NO_(x).

The blower in the recycling passage renders the recycling possibleregardless of the pressure ratios between the intake and exhaust sidesof the engine, which means that recycling may be effected at any desiredengine load.

The exhaust gas is diverted upstream of the turbine of the turbochargerand is cooled and purified by means of the addition of water. The blowerdischarges the recycled gas downstream of the compressor of theturbocharger. The dimensions of the turbine and compressor of theturbocharger may be reduced because the recycled gas does not have to becompressed in the compressor. The preferred cooling of the gas upstreamof the blower reduces the energy consumption of the blower to abouthalf, judged in relation to a situation where the blower would have tocompress uncooled exhaust gas. Furthermore, the sensitive blower is notsubjected to the corrosive products in the exhaust gas.

The purification of the recycled gas may be promoted by removing waterdrops from the exhaust gas after the addition of atomized water at eachstage. The water addition in a second stage results in a drop of the gastemperature, which causes condensation of water from the 100 percenthumidity-saturated gas. The condensation will occur first on theparticulate impurities to be found in the gas. When the water drops arethen removed from the gas before it leaves the stage, the undesiredimpurities will be removed with the drops. If the temperature of the gasthus purified is lowered further by addition of water in a subsequentstage, this temperature drop will lead to condensation from the gas ofmore than the last added amount of water, and this water will be largelypure. This means that the gas may be purified in the scrubber withoutany particular water consumption.

In a particularly suitable embodiment, the scrubber is connected withthe charging air conduit and cools both the air compressed by thecompressor and the recycled exhaust gas. Thus, the well-known pipecooler for cooling the charging air may be dispensed with. This resultsin an advantageous simplification of the intake system of the engine,but it is also possible, with the direct water cooling of the chargingair and the recycled exhaust gas contained therein, to obtain a gastemperature at the intake to the combustion chambers which is from 10°to 12° C. lower than the temperature obtainable with a pipe cooler.

Other things being equal, such a reduction of the charging airtemperature results in an improvement of the specific fuel oilconsumption (SFOC) of about 1 g/hph. The lower charging air temperaturealso reduces the maximum combustion temperature and thus the NO_(x)content of the exhaust gas. The above advantages of the invention arefurther exploited fully, because all the air passed to the combustionchamber is humidified under all operation conditions for the engine.Thus the NO_(x) amount is about halved. If the engine is used on a ship,an additional result is that the charging air is purified of the saltcontent which is often found in the intake air, whereby the engine isnot subjected to corrosive influences from the salt.

In a preferred embodiment, which is particularly useful in a ship, thescrubber has a first humidification stage wherein atomizer nozzlesatomize sea water in the through-flowing gas, an intermediatepurification stage wherein the liquid drops suspended in the gas areseparated from the gas, and at least one final cooling stage whereinatomizer nozzles atomize fresh water in the through-flowing gas, andfrom which condensed fresh water is removed.

Very large amounts of water must be used for humidification andpurification of the exhaust gas. Typically, the water consumption isfrom 3 to 4 times larger than the fuel oil consumption of the engine. Ina large medium-size diesel engine with an output of 35,000 h.p., thisresults in a water Consumption of 12-17 tonnes per hour. On board a shipit is relatively energy-consuming to produce fresh water, which isusually done in special freshwater generators converting the sea waterinto fresh water by evaporation at low pressure.

In the preferred embodiment, the problem of freshwater consumption issolved by humidification and purification in several stages, and by theuse of sea water in the first stage, and fresh water in at least thelast water addition stage. The sea water is available in unlimitedamounts, and, as mentioned above, the salt will be removed from the gastogether with the other pollutants. If water is also added in a thirdand optionally in a fourth stage, the temperature drop of the gas causedthereby will result in a separation of fresh water which by far exceedsthe added amount of fresh water. In addition to covering the waterrequirement for the purification, it is possible to obtain the furtheradvantage of an actual production of fresh water which may be usedelsewhere in the ship.

Examples of embodiments according to the invention will now be explainedbelow in greater detail with reference to the very schematic drawing, inwhich

FIGS. 1 and 2 illustrate diagrams of two different embodiments of theintake and exhaust systems of an internal combustion engine according tothe invention, and

FIG. 3 is an outline of a stage in a scrubber.

In the three embodiments, the internal combustion engine is generallydesignated 1. The engine has a charging air receiver 2 and an exhaustreceiver 3, and the exhaust valves belonging to the combustion chambersare indicated by 4. The engine may be a large two-strokeconstant-pressure-charged diesel engine, which may be used as the mainengine in a ship or as a stationary engine for operating a generator ina power station. The total output of the engine may, for example, rangefrom 5,000 to 70,000 kW, but the invention may also be used infour-stroke engines with an output of, for example, 1,000 kW.

The charging air is passed from the charging air receiver to theindividual cylinders and scavenging air ports. When the exhaust valve 4is opened, the exhaust gas flows through an exhaust conduit into theexhaust receiver 3 and onwards through a conventional exhaust conduit 5to a turbine 6 of a turbocharger, from which the exhaust gas flows awaythrough the exhaust conduit 7. Via a shaft 8, the turbine 6 drives acompressor 9 supplied via an air intake 10. The compressor deliverspressurized charging air to a charging air conduit 11 leading to thecharging air receiver 2.

The embodiment shown in FIG. 1 is particularly relevant in cases wherean existing engine is rebuilt for operation according to the inventionwithout replacement of the existing tube cooler 12 for cooling thecharging air. The tube cooler has a cooling water inlet 13 and a coolingwater outlet 14. Normally, such a tube cooler can only cool the chargingair to a temperature about 15° C. above the cooling water temperature.

A recycling conduit 15 branches off from the exhaust conduit 5 and isconnected with the charging air conduit 11 so that the recycling conduitforms a recycling passage together with the part of the intake systemwhich is downstream of the connection of the conduit 15 to the conduit11.

A scrubber 16 has been inserted in the conduit 15 for humidification andpurification of the exhaust gas with water. The gas outlet from thescrubber 16 leads to a blower 17 driven by an engine 18. Even though theexhaust gas is diverted at the high-pressure side of the turbine 6, theblower 17 has to provide it with an increase in pressure which increasesthe pressure in the recycled exhaust gas to above the charging airpressure in the conduit 11. The scrubber 16 may have one or more stagesdepending on the degree of purification and cooling of the exhaust gas.

The second embodiment shown in FIG. 2 differs from the first embodimentin that the tube cooler 12 has been replaced by a water scrubber 19having four stages. In a first stage 20, pressurized sea water from apipe 21 is passed to atomizer nozzles which humidify and cool the gaswith a suitable excess of water so that no salt particles areprecipitated in the gas. The end of the first stage contains a dropcollector which separates the liquid drops suspended in the gas andcontaining the undesired pollutants from the gas. In a second stage 29,fresh water supplied through a pressurized pipe 22 is atomized into thegas and thus causes a further cooling with subsequent water condensationon any remaining particles therein. A third stage 23 contains a dropcollector which removes the liquid drops suspended in the gas. Through apipe 24, the separated water is passed overboard or to a tank for laterpurification. Even if the water is passed overboard, it is possible toobtain an environmental advantage as the undesired substances in theexhaust gas are thus deposited directly in the sea without having to govia the atmosphere. If desired, the water may be purified with slakedlime and optionally filtered before discharge. In a fourth stage 25,fresh water supplied through a pipe 26 is atomized into the gas, which,as mentioned above, causes condensation of an even larger amount offresh water which is removed from the scrubber through a pipe 27, thegas temperature being brought to very close to the inlet temperature ofthe water. The scrubber may have fewer stages, but then the cooling ofthe gas will be less.

FIG. 3 shows an example of the embodiment of a stage 33 in a scrubberfor arranging in the recycling passage. The housing 34 of the scrubbermay be connected with the gas-transporting conduit by means of flanges35 or with further scrubber stages. The gas flow direction is indicatedby arrows 36. An inlet pipe 37 for fresh or sea water leads the water todistributor tubes 38 each carrying a number of nozzles 39 which atomizethe water into the gas. In the embodiments shown, the nozzles spray thewater out against the direction of flow of the gas, which gives a goodevaporation and purification effect, but it is, of course, also possibleto use nozzles spraying in the direction of flow of the gas. Afterpassage of the nozzles, the gas flows through a drop generator 40containing large surfaces which are angled in relation to the directionof flow of the gas. The material of the drop generator may, for example,consist of very porous foam, wire mesh or steel wool. In the dropgenerator, small liquid drops in the gas will join together into largerdrops, which facilitates the subsequent removal of the drops from thegas in a drop collecting section 41 which, as shown, may containelongated plate sections 42 which are inclined in relation to thedirection of flow of the gas so that the gas is forced to change itsdirection upon passage of the plates 42. As a consequence of the inertiaof the drops, they will be slower to change their direction than the gasitself, and the drops will therefore land on the plates 42 and slidealong them into a folded-over end portion 43 where the drops are caughtand passed to a collecting channel discharging the condensate and theremoved liquid to a discharge pipe 44. It will often be possible to omitthe drop generator 40 which may cause a certain flow resistance. If thescrubber has several consecutive atomizer stages, each stage preferablycontains a drop collecting section so that subsequent stages do not haveto cool the amount of liquid removed in the drop collecting section. Ifthe scrubber only has to purify the recycled amount of gas, it is oftenpossible to obtain sufficient purification in a single scrubber stage.The scrubber only yields low flow resistance and requires substantiallyno maintenance and is also cheap to manufacture.

The below examples describe the mode of operation of the scrubber in anengine plant where the recycled gas is purified in a scrubber and allthe intake air is cooled in a two-stage scrubber. For the sake ofsimplicity, calculations are based on an engine of a full-load output of10,000 kW and a nominal charging air pressure of 3.55 bar.

EXAMPLE 1

The engine is run at 100 percent load, and ambient air has a temperatureof 25° C. and a relative humidity of 30 percent, which means that theintake air will contain about 6 g of water/kg of air. The airconsumption of the engine is about 22 kg/s. After the compressor 9, thetemperature of the air is T₁ =185° C.

In the first scrubber stage, the atomizer nozzles are supplied with anamount of salt water of 2.6 l/s, whereby the air is cooled byevaporation to a temperature of about T₁ =70° C., and the air is at thesame time humidified to 100 percent relative humidity, resulting in awater content of 60 g/kg of air. In the drop collector, a water amountof about 1.3 l/s is removed. At discharge from the first scrubber stage,the air is largely purified of any salt content.

In the second scrubber stage, a fresh water amount of 35 l/s is sprayedout through the atomizer nozzles at a water temperature of about T_(v)=25° C. This cools the air to a temperature of about T₁ =35° C., wherethe 100 percent humidity-saturated air has a water content of about 9g/kg of air. In the drop collector, a water amount of about 36.1 l/s isseparated, whereby the second scrubber stage produces an amount of freshwater of 1.1 l/s, corresponding to about 95 tons per day. Extremely highcoefficients of heat transfer have been measured at atomization coolingof the intake air, and it is estimated that the coefficients of heattransfer are about 50-100 times as high as in conventional pipe coolers.If desired, the air in a third scrubber stage may be cooled to a fewdegrees above water temperature, which will result in a furtherproduction of fresh water.

If a separate scrubber 16 is used for purification of the recycledexhaust gas in the conduit 15, it will be suitable, simultaneously withthe purification, to cool the exhaust gas from the inlet temperature ofabout 375° C. to the temperature of the intake air of T₁ =185° C. afterthe compressor. The water consumption in the scrubber 16 will be about0.07 l/s, corresponding to 5.7 tonnes per day. This consumption may becovered by fresh water from the second scrubber without any problems. Ifa pipe cooler is used instead of said scrubber, the scrubber 16 may beoperated with sea water.

EXAMPLE 2

With the same ambient conditions as above and an engine load of 75percent, the atomizer nozzles in the first scrubber stage have to besupplied with an amount of sea water of at least 1.8 l/s, whereby theair is cooled to about T₁ =60° C. and humidified to 100 percent relativehumidity, where the water content is at about 45 g/kg of air.

At the second scrubber stage, a water amount of 35 l/s is supplied at atemperature of about T_(v) =25° C., whereby the gas is cooled to aboutT₁ =30° C., where the water content in the 100 percent saturated gas isabout 12 g/kg of air. The second stage produces an amount of fresh waterof about 0.5 l/s corresponding to 43 tonnes per day.

Purification of the recycled exhaust gas consumes an amount of water ofabout 0.10 l/s, corresponding to 8.6 tonnes per day.

Instead of the scrubbers described above, it is possible to use ascrubber known from an inert gas plant of the Japanese make GadeliusMarine K. K. and a drop collector from the same firm acting according tothe cyclone principle. However, these known systems have thedisadvantage of being bulky.

We claim:
 1. A large supercharged diesel engine (1), such as a mainengine of a ship, comprising a turbocharger having an exhaust gas-driventurbine (6) and a compressor (9) driven by the turbine and supplyingcharging air to the engine cylinders, and a recycling passage forreturning part of the exhaust gas to the engine cylinders, characterizedin that the recycling passage (15, 11) includes a unit forhumidification of the exhaust gas with water, that the humidificationunit is a scrubber (16, 19) having a number of water atomizer stages(20, 29, 25), that the recycling passage comprises a conduit (15) whichbranches off from the exhaust conduit (5) upstream of the turbine (6)and is connected with the charging air conduit (11) downstream of thecompressor (9), and that the conduit is connected with the scrubber (16)and with a blower (17), for augmentation of the pressure in the recycledexhaust gas.
 2. A diesel engine according to claim 1, characterized inthat the scrubber (19) is connected with the charging air conduit (11)so that it cools both the air compressed by the compressor and therecycled exhaust gas.
 3. A diesel engine according to claim 1characterized in that the scrubber hasa first humidification stage (20)wherein atomizer nozzles atomize sea water in the through-flowing gas,an intermediate purification stage (23) wherein the liquid dropssuspended in the gas are separated from the gas, and at least one finalcooling stage (25) wherein atomizer nozzles atomize fresh water in thethrough-flowing gas, and from which condensed fresh water is removed.